Experimental investigation of the mitigation of bubble collapse loads due to the free surface

Small bubbles were generated beneath the free surface using the electric spark method. Their behavior was observed through high-speed photography, and the impact forces were measured using a polyvinylidene fluoride (PVDF) sensor attached to the vertical solid wall and horizontal wall above an air pocket in a groove plate. The proximity parameter γ_w and γ_f were defined as the non-dimensionalization for the free surface and vertical wall, respectively, and two-dimensional map of the impact load and jet orientation was presented based on the parameters. The findings indicate that the position of the bubble center, where the maximum force is generated as it approaches the free surface, is located about two times farther from the vertical wall compared to the region where the free surface has no influence. Additionally, the jet direction is directed toward the lower side of the water surface and perpendicular to the wall. Moreover, the impact force on the wall was also measured when the air gap existed between the bubble and the wall. In this case, the measured force was significantly smaller compared to when no air gap was present. The experimental data suggests that the presence of an air gap can effectively reduce the impact force generated by the bubble which smaller than the air gap size.